Electrode catalyst with improved longevity properties and fuel cell using the same

ABSTRACT

Disclosed is an electrode catalyst comprising: (a) a support; (b) metal catalyst particles supported on the support and formed of a catalytically active metal or metal-containing alloy; and (c) an anti-coarsening compound, which is dispersed in at least one region selected from the group consisting of interstitial spaces among the catalyst particles and contact sites between the support and the catalyst particles, and has a coarsening temperature higher than that of the catalyst. A method for preparing the electrode catalyst is also disclosed. Additionally, disclosed is a method for preventing metal catalyst particles supported on a support and formed of a catalytically active metal or metal-containing alloy from coarsening, the method comprising: dispersing an anti-coarsening compound having a coarsening temperature higher than that of the metal catalyst, in at least one region selected from the group consisting of interstitial spaces among the metal catalyst particles and contact sites between the support and the metal catalyst particles. The electrode catalyst is structurally stable while not causing degradation of electrochemical quality, and thus can improve the longevity properties of a fuel cell.

This application claims the benefit of the filing date of Korean PatentApplication No. 10-2005-0065206, filed on Jul. 19, 2005 in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirely by reference.

TECHNICAL FIELD

The present invention relates to an electrode catalyst that provides afuel cell with excellent lifespan characteristics by preventing metalcatalyst components from coarsening due to variations in temperaturewhile not adversely affecting the electrochemical quality. Also, thepresent invention relates to a method for preparing the same electrodecatalyst and a fuel cell comprising the same electrode catalyst.

BACKGROUND ART

Recently, active and intensive studies of fuel cells as next-generationenergy sources have been conducted since fuel cells are pollution-freeclean energy sources so that they can substitute for other existingenergy sources. The basic concept of a fuel cell is the use of electronsgenerated by the reaction of hydrogen with oxygen. A fuel cell isdefined as a cell capable of producing direct current by convertingchemical energy, derived from a chemical reaction of fuel gas includinghydrogen with an oxidant including oxygen, directly into electricenergy. Unlike other conventional batteries, fuel cells generateelectricity by utilizing fuel and air supplied from the exterior. Fuelcells may be classified depending on drive conditions into phosphoricacid fuel cells, alkaline fuel cells, proton exchange membrane fuelcells, molten carbonate fuel cells, direct methanol fuel cells and solidelectrolyte fuel cells. Particularly, proton exchange membrane fuelcells (PEMFC) have a high energy density and can be used at roomtemperature, and thus have been in the spotlight as portable electricpower sources.

In a proton exchange membrane fuel cell (PEMFC), protons generated inthe anode are transferred to the cathode through a polymer electrolytemembrane, thereby forming water via the bonding of oxygen and electrons.A PEMFC utilizes the electrochemical energy generated at this time.Because a PEMFC is driven at a low temperature, it shows a relativelylow efficiency when compared to other fuel cells. Therefore,platinum-supported carbon is generally used as a catalyst in a PEMFC inorder to increase the efficiency of the fuel cell. In fact, use of aplatinum-supported carbon catalyst provides a fuel cell with a markedlyimproved quality when compared to fuel cells using other metal-supportedcatalysts.

However, because platinum supported on a support of theplatinum-supported carbon used as a catalyst for a proton exchangemembrane fuel cell merely has a size of several nanometers, the catalystis unstabilized as electrochemical reactions proceed and coarsening ofplatinum nanoparticles occur. Such coarsening of platinum nanoparticlesgradually causes a drop in surface area of platinum nanoparticlesrequired for the reactions. This also adversely affects the quality of afuel cell.

DISCLOSURE OF THE INVENTION

Therefore, the present invention has been made in view of theabove-mentioned problems. The inventors of the present invention havefound that when a compound capable of preventing metal ormetal-containing alloy particles from coarsening is coated onto and/ordispersed into interstitial spaces among catalytically active componentspresent on a support, such as platinum or platinum-containing particles,and/or contact sites between the support and the metal particles, it ispossible to prevent the electrode catalyst components from coarseningwhile not adversely affecting the electrochemical quality of a fuelcell, and thus to improve the lifespan characteristics of the fuel cell.

Therefore, it is an object of the present invention to provide anelectrode catalyst having excellent lifespan characteristics due to itsstructural stability, a method for preparing the same electrodecatalyst, and a fuel cell comprising the same electrode catalyst.

It is another object of the present invention to provide a method forpreventing a metal catalyst component supported on a support fromcoarsening by enhancing the structural stability of the metal catalystcomponent.

According to an aspect of the present invention, there is provided anelectrode catalyst comprising: (a) a support; (b) metal catalystparticles supported on the support and formed of a catalytically activemetal or metal-containing alloy; and (c) an anti-coarsening compound,which is dispersed in at least one region selected from the groupconsisting of interstitial spaces among the metal catalyst particles andcontact sites between the support and the metal catalyst particles, andhas a coarsening temperature higher than that of the metal catalyst.Also, the present invention provides a method for preparing the sameelectrode catalyst, a membrane electrode assembly (MEA) comprising thesame electrode catalyst, and a fuel cell, preferably a proton exchangemembrane fuel cell (PEMFC) comprising the same membrane electrodeassembly.

According to another aspect of the present invention, there is provideda method for preventing metal catalyst particles supported on a supportand formed of a catalytically active metal or metal-containing alloyfrom coarsening, the method comprising: dispersing an anti-coarseningcompound having a coarsening temperature higher than that of the metalcatalyst, in at least one region selected from the group consisting ofinterstitial spaces among the metal catalyst particles and contact sitesbetween the support and the metal catalyst particles.

Hereinafter, the present invention will be explained in more detail.

According to the present invention, a compound (anti-coarseningcompound) having a coarsening temperature higher than that of thecatalytically active component is dispersed onto the electrode catalyst(e.g. platinum-supported carbon) used in a fuel cell, wherein thecompound is dispersed in and coated onto specific sites capable ofinhibiting the metal catalyst particles from coarsening, such specificsites including interstitial spaces among the metal catalyst particlesand contact sites between the support and the metal catalyst particles(see FIG. 1). The term “coarsening temperature” refers to thetemperature where crystal particles start to grow.

By virtue of the above characteristics of the present invention, it ispossible to obtain the following effects.

A conventional catalytically active component, including a metal such asplatinum, is generally provided in the form of particles having a smalldiameter of several nanometers so as to show excellent catalyticactivity via an increased specific surface area. Such catalyticallyactive component particles become unstable as electrochemical reactionsproceed, thereby causing a so-called coarsening phenomenon includingagglomeration of the particles. Such coarsening causes a drop in surfacearea of catalyst particles needed for the reactions, resulting indegradation of the quality of a fuel cell using the catalyst. To solvethis problem, a certain compound (anti-coarsening compound) has beenintroduced onto the surface of the catalyst particles. However, presenceof such anti-coarsening compounds on the surface of the metal catalystparticles causes an increase in electric resistance among the metalcatalyst particles and a drop in proton conductivity to the surface ofthe metal catalyst particles. In addition to this, the surface of themetal catalyst particles to be used in catalytic reactions is poisonedby the anti-coarsening compounds and the catalyst shows a decreasedreactive surface, resulting in degradation of the quality of a fuelcell.

On the contrary, according to the present invention, the anti-coarseningcompound is not coated on the surface of the catalytically activeparticles but is coated onto and/or dispersed in specific sites capableof inhibiting the metal catalyst particles from coarsening to thehighest degree, such specific sites including interstitial spaces amongthe metal catalyst particles and contact sites between the support andthe metal catalyst particles. Therefore, even if the catalyticallyactive particles become unstable during electrochemical reactions, it ispossible to inhibit the metal catalyst particles from coarsening.Accordingly, it is possible to increase the thermal and structuralstability of an electrode catalyst, to minimize degradation of thequality of a fuel cell, and to improve the lifespan characteristics of afuel cell. In fact, it can be seen from the following experimentalexamples that coarsening of nanoparticles of a catalytically activemetal or metal alloy is significantly reduced according to the presentinvention.

(1) According to the present invention, the anti-coarsening compound,which is dispersed uniformly in interstitial spaces among the electrodecatalyst, for example catalyst particles formed of a catalyticallyactive metal or metal-containing alloy supported on a support, and/or incontact sites between the support and the catalyst particles so as toimprove the structural stability of the electrode catalyst, includes anycompound having a coarsening temperature higher than that of the metalcatalyst particles formed of the catalytically active metal ormetal-containing alloy, with no particular limitation. For example, mostnoble metal elements initiate coarsening at a temperature of 300° C. orlower, thereby causing degradation of the quality and lifespancharacteristics of a fuel cell. Thus, an anti-coarsening compound havinga coarsening temperature of at least 300° C. is preferred.

(2) Additionally, it is preferable that the anti-coarsening compoundcauses no degradation in the catalytic activity, i.e. electrochemicalquality, of the noble metal or metal-containing alloy particles.Particularly, it is preferable that the anti-coarsening compound has ahigher affinity to the catalyst support as compared to its affinity tothe metal catalyst component.

Herein, the term “affinity” refers to a degree of deposition (coating)of the anti-coarsening compound. In other words, because the surface ofcarbon and that of the metal (e.g. platinum) particles have differenthydrophilicity and redox capability, an anti-coarsening compound havinga different degree of affinity to those surfaces showing differentelectrochemical properties can be positioned selectively in specificsites instead of the metal surface, such sites including interstitialspaces among the metal catalyst particles and contact sites between thesupport and the metal catalyst particles.

Non-limiting examples of the compound having the above characteristicsinclude metal phosphates, metal oxides, metal nitrides, metal fluorides,metal carbides, or the like, wherein the metal includes a conventionalmetal known to those skilled in the art, for example, an alkali metal,an alkaline earth metal, a Group 13 element in the Periodic Table, aGroup 14 element in the Periodic Table, a transition metal, or the like.

Particular examples of such compounds include aluminum phosphate-basedcompounds, zirconium oxide, cerium oxide, silicon oxide, aluminum oxideor a mixture thereof. Among these compounds, metal phosphate-basedcompounds are insulators, while they have a loose structure and a smallthickness, so that they do not interrupt diffusion of materials and theycause little drop in electrochemical quality of an electrode catalyst.

The anti-coarsening compound is dispersed on the surface ofcatalytically active metal or metal-containing alloy particles,interstitial spaces among the metal catalyst particles and/or contactsites between the support and the metal catalyst particles. In order toeffectively prevent the metal catalyst particles from coarsening, theanti-coarsening compound is dispersed preferably in interstitial spacesamong the metal catalyst particles and/or contact sites between thesupport and the metal catalyst particles. There are no particularlimitations in the shape and thickness of the compound dispersed in theabove sites, and the shape and thickness are controllable in such arange that the metal or metal-containing alloy particles can beprevented from coarsening. Preferably, the anti-coarsening compound isdispersed in a thickness of 1˜5 mm.

There is no particular limitation in the main catalytically activecomponent (b) forming the electrode catalyst according to the presentinvention, as long as it allows the oxidation of hydrogen or reductionof oxygen. The catalytically active component (b) includes any metal ormetal-containing alloy generally known to those skilled in the art.Particularly, noble metals, for example platinum (Pt) or Pt-containingalloys are preferred. Herein, non-limiting examples of the metal thatforms an alloy with platinum include ruthenium (Ru), rhodium (Rh),palladium (Pd), gold (Au), silver (Ag), iridium (Ir), osmium (Os) or amixture thereof.

Although there is no particular limitation on the size (particlediameter) of the metal catalyst particles, the metal catalyst particleshave a diameter preferably of 1˜10 nm, more preferably of 1.5˜5 nm.Additionally, the electrode catalyst comprising the catalytically activemetal particles may be supported on a support known to those skilled inthe art. The electrode catalyst comprising the metal component alone isalso included in the scope of the present invention.

The support (a) forming the electrode catalyst according to the presentinvention is used to allow the noble metal catalyst to be dispersedwidely in a broad surface area of the support and to improve physicalproperties including thermal and mechanical stability, which otherwisecannot be obtained by using the metal catalyst alone. For example, themetal catalyst may be coated on a support formed of microparticlescurrently used in the art. Any suitable methods other than the abovecoating method may also be used. Particular examples of the support thatmay be used in the present invention include porous carbon, conductivepolymers or metal oxides.

Porous carbon that may be used as a support according to the presentinvention includes active carbon, carbon fiber, graphite fiber or carbonnanotubes, and conductive polymers that may be used in the presentinvention include polyvinyl carbazole, polyaniline, polypyrrole or aderivative thereof. Additionally, at least one metal oxide selected fromthe group consisting of oxides of tungsten, titanium, nickel, ruthenium,tantalum and cobalt may also be used as a support.

Although there is no particular limitation on the size of the support,the support has a size preferably of 0.01˜10 μm, and more preferably of0.05˜0.5 μm.

The electrode catalyst can be obtained by using a conventional method,except that an anti-coarsening compound is dispersed uniformly inspecific sites of the electrode catalyst. One embodiment of the methodfor preparing the electrode catalyst according to the present inventioncomprises the steps of: (a) dispersing or dissolving an anti-coarseningcompound having a coarsening temperature higher than that of acatalytically active metal or metal-containing alloy into a solvent toprovide a dispersion or solution; (b) adding a support, on which metalcatalyst particles formed of the catalytically active metal ormetal-containing alloy are supported, into the dispersion or solutionobtained from step (a) so as to be coated with the dispersion orsolution, followed by drying; and (c) heat treating the dried productobtained from step (b).

(1) First, a coating solution, in which a compound having a coarseningtemperature higher than that of a catalytically active metal ormetal-containing alloy is dispersed or dissolved, is prepared. Herein,the coating solution includes both a solution and a homogeneoussuspension.

The anti-coarsening compound that may be used in the method is the sameas defined hereinbefore. As the solvent, any solvent capable ofdissolving or dispersing the above compound may be used, and distilledwater is particularly preferred. For example, the coating solution maybe prepared by dissolving an aluminum phosphate-based compound, orprecursor compounds each containing aluminum and phosphoric acid, suchas aluminum nitrate (Al(NO₃)₃.9H₂O)) and ammonium phosphate((NH₄)₂HPO₄), into distilled water.

(2) Next, a support on which the metal catalyst particles are supportedis introduced into the coating solution so as to be coated with thecoating solution, followed by drying.

The catalytically active metal or metal-containing alloy supported onthe support may be prepared by a method currently used in the art, forexample, a precipitation method or a colloid method. There is noparticular limitation in the method. For example, a catalytically activemetal precursor or a metal-containing precursor compound is added to andallowed to react with a support dispersion obtained by dispersing asupport into a solvent, optionally with a reducing agent and a pHmodifier, and then the resultant powder is dried.

When the catalyst particle-supported support is added to the coatingsolution (dispersion or solution), the metal catalyst particles and theanti-coarsening compound are used suitably in a molar ratio of 1˜5:1,preferably of 2˜3:1. If the molar ratio is less than 1:1, the metalcatalyst particles are distributed in an excessively low amount, therebyproviding low catalytic activity. In other words, when the concentrationof the anti-coarsening compound is too high, the compound may be presenteven on the surface of the metal catalyst particles, besides thespecific sites suitable for inhibiting the particles from coarsening tothe optimum degree, i.e., interstitial spaces among the metal catalystparticles and/or contact sites between the support and the metalcatalyst particles. Therefore, in this case, presence of theanti-coarsening compound on the surface of the metal catalyst particlescauses the problems as described above and consequently leads to thedegradation of the quality of a fuel cell, such problems including anincrease in electric resistance among catalyst particles, degradation ofthe proton conductivity to the surface of the metal catalyst particles,and a decrease in reactive surface area of the catalyst. If the molarratio is greater than 5:1, the metal catalyst particles are present inan excessively large amount when compared to the anti-coarseningcompound. Thus, it is not possible to obtain the anti-coarsening effectsufficiently.

The coating step may be performed by using a conventional methodgenerally known to those skilled in the art. Additionally, there is noparticular limitation in the drying method. However, it is preferredthat the coated catalyst particles are dried at a temperature of 90° C.or lower for several hours to allow distilled water used in the coatingsolution to evaporate completely.

(3) After the anti-coarsening compound is dispersed on the surface ofthe catalytically active metal or metal-containing alloy particlessupported on the support or interstitial volumes among the particles toa desired thickness, the electrode catalyst is heat treated to completethe structurally stable electrode catalyst according to the presentinvention.

Such heat treatment further stabilizes the structure of theanti-coarsening compound in the electrode catalyst and the bindingbetween the support and the anti-coarsening compound. Additionally, suchheat treatment also serves to completely remove a trace amount ofimpurities that may inhibit electrochemical reactions under the driveconditions of the electrode catalyst. Herein, the heat treatment stepmay be performed at any temperature with no particular limitation, aslong as the temperature is lower than the temperature where the metalcatalyst particles formed of the catalytically active metal ormetal-containing alloy start coarsening. For example, the heat treatmentstep may be performed at a temperature ranging from 110° C. to 300° C.for 2˜4 hours.

The method for preparing the electrode catalyst according to the presentinvention includes the use of an aqueous coating solution and comprisesa simple process requiring a relatively short time of heat treatment, sothat the method can be applied to an existing process with ease and canbe performed at a reduced cost.

According to another aspect of the present invention, there is providedan electrode for fuel cells, which comprises the electrode catalystobtained as described.

The electrode for fuel cells comprises a gas diffusion layer and acatalyst layer. It may comprise a catalyst layer alone. Otherwise, itmay have a catalyst layer integrally formed on a gas diffusion layer.

The electrode for fuel cells according to the present invention can bemanufactured by a conventional method known to one skilled in the art.In one embodiment of the method, the electrode catalyst is mixed withcatalyst ink that contains a highly proton conductive polymer materialand a mixed solvent enhancing dispersion of the catalyst to provideslurry. Then, the slurry is applied on carbon paper by a printing,spraying, rolling or a brushing process, and then dried.

According to still another aspect of the present invention, there isprovided a membrane electrode assembly (MEA) for fuel cells, whichcomprises: (a) a first electrode having a first catalyst layer; (b) asecond electrode having a second catalyst layer; and (c) an electrolytemembrane interposed between the first electrode and the secondelectrode, wherein either or both of the first catalyst layer and thesecond catalyst layer comprise the electrode catalyst according to thepresent invention.

One of the first and the second electrodes is a cathode and the other isan anode.

The membrane electrode assembly refers to an assembly of an electrodefor carrying out an electrochemical catalytic reaction between fuel andair with a polymer membrane for carrying out proton transfer. Themembrane electrode assembly is a monolithic unit having a catalystelectrode adhered to an electrolyte membrane.

In the membrane electrode assembly, each of the catalyst layers of theanode and cathode is in contact with the electrolyte membrane. The MEAcan be manufactured by a conventional method known to one skilled in theart. For example, the electrolyte membrane is disposed between the anodeand cathode to form an assembly. Next, the assembly is inserted into thegap between two hot plates operated in a hydraulic manner whilemaintaining a temperature of about 140° C., and then pressurized toperform hot pressing.

There is no particular limitation in the electrolyte membrane, as longas it is a material having proton conductivity, mechanical strengthsufficient to permit film formation and high electrochemical stability.The electrolyte membrane includes, but not exclusively,tetrafluoroethylene-co-fluorovinyl ether, wherein the fluorovinyl ethermoiety serves to transfer protons.

According to yet another aspect of the present invention, there isprovided a fuel cell comprising the above membrane electrode assembly.

All materials forming the fuel cell, other than the electrode catalystaccording to the present invention, are those currently used in aconventional fuel cell. Particularly, all materials used in a protonexchange membrane fuel cell, are preferred. Also, there is no particularlimitation in the method for manufacturing the fuel cell. The fuel cellmay be manufactured by using the above membrane electrode assembly,which comprises a cathode, an anode and a membrane electrode assemblycomprising an electrolyte coated with an active layer containing theelectrode catalyst, and a bipolar plate in a conventional manner knownto one skilled in the art.

Preferably, the fuel cell is a proton exchange membrane fuel cell(PEMFC) that adopts reduction of oxygen and oxidation of hydrogen, butis not limited thereto.

Further, the present invention provides a method for preventing metalcatalyst particles supported on a support and formed of a catalyticallyactive metal or metal-containing alloy from coarsening, the methodcomprising: dispersing an anti-coarsening compound having a coarseningtemperature higher than that of the metal catalyst, in at least oneregion selected from the group consisting of interstitial spaces amongthe metal catalyst particles and contact sites between the support andthe metal catalyst particles.

In fact, it can be seen from the following experimental examples thatcatalytically active metal or metal-containing alloy particles can beprevented from coarsening even under an increased temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic view showing the condition of the surface of anelectrode catalyst according to the present invention;

FIG. 2 is a schematic view showing a proton exchange membrane fuel cell;

FIG. 3 is a graph showing the results of X-ray diffraction analysis ofthe platinum-supported carbon catalysts coated with an aluminumphosphate-based compound according to Examples 1 and 2, and of thenon-coated platinum-supported carbon catalyst according to ComparativeExample 1; and

FIG. 4 is a graph showing the quality of a fuel cell using theplatinum-supported carbon catalyst coated with an aluminumphosphate-based compound according to Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only, and the scope of the present invention is notlimited thereto.

EXAMPLES 1 AND 2 Example 1

1-1. Preparation of Platinum-Supported Carbon Catalyst Coated withAluminum Phosphate-Based Compound

Ammonium phosphate was added to a predetermined amount of distilledwater and the mixture was agitated sufficiently to dissolve the ammoniumphosphate therein. Next, aluminum nitrate was added thereto and themixture was agitated sufficiently. At this time, ammonium phosphate andaluminum nitrate were used in a weight ratio of 0.38:1. As the reactionprogressed, a white-colored coating solution was obtained while analuminum phosphate-based compound was formed. To the coating solution,platinum-supported carbon containing platinum particles having anaverage particle size of about 2.6 nm was added. At this time, platinumand an aluminum phosphate-based compound were used in a molar ratio of3:1. The resultant coated platinum-supported carbon was completely driedin an oven at a temperature of 90° C. or less, and then heat treated at200° C. for 2 hours to cause the aluminum phosphate-based compound to becoated on the surface of the platinum-supported carbon.

1-2. Manufacture of Electrode

The platinum-supported carbon coated with the aluminum phosphate-basedcompound according to Example 1-1 was mixed with Nafion ink having highproton conductivity, and the mixture was applied onto a carbon electrodeto provide an oxygen electrode. Additionally, a catalyst (Hispec 9100)commercially available from Johnson-Matthey Co. was used in the samemanner as described above to provide a fuel electrode. Herein, platinumwas supported in an amount of 0.25 mgPt/cm² and 0.5 mgPt/cm²,respectively, in the oxygen electrode and the fuel electrode.

1-3. Manufacture of Membrane Electrode Assembly (MEA)

A Nafion electrolyte membrane was bonded thermally and mechanicallybetween both electrodes obtained according to Example 1-2 to provide amembrane electrode assembly, which, in turn, was used to manufacture aproton exchange membrane fuel cell (see FIG. 2).

Example 2

An electrode catalyst, a membrane electrode assembly and a protonexchange membrane fuel cell were manufactured in the same manner asdescribed in Example 1, except that the heat treatment was performed ata temperature of 300° C. instead of 200° C.

Comparative Example 1

A membrane electrode assembly and a fuel cell comprising the same weremanufactured in the same manner as described in Example 1, except that aplatinum-supported carbon catalyst that was not coated with an aluminumphosphate-based compound was used.

Experimental Example 1 X-Ray Diffraction Analysis for Electrode Catalyst

Analysis for the electrode catalyst according to the present inventionwas performed as follows.

The platinum-supported carbon catalysts coated with an aluminumphosphate-based compound were used as samples, and non-coatedplatinum-supported carbon was used as a control.

FIG. 3 shows the results of the X-ray diffraction analysis for thecrystal structure of platinum supported on a support, wherein the widthof the Pt(111) peak relates to the size of platinum particles, andplatinum particle size decreases as the peak width increases. In thecase of the electrode catalyst heat treated at 200° C. according toExample 1, it can be seen that growth of platinum particles, i.e.coarsening of platinum particles is inhibited by the coating effect ofthe metal phosphate compound used as an anti-coarsening compound.Additionally, in the case of the electrode catalyst heat treated at 300°C. according to Example 2 shows a slightly decreased peak width. Thisindicates that partial coarsening of platinum particles occurs (see FIG.3).

Experimental Example 2 Evaluation for Coarsening Tendency of ElectrodeCatalyst

The following test was performed to evaluate the coarsening tendency ofthe electrode catalyst according to the present invention.

The platinum-supported carbon catalyst coated with an aluminumphosphate-based compound according to Example 1 was used as a sample,and the non-coated platinum-supported carbon catalyst according toComparative Example 1 was used as a control. The platinum nanoparticlespresent in each electrode catalyst were measured for variations in sizewhile varying the temperature from room temperature to 300° C. Theresults are shown in the following Table 1.

After the test, the non-coated conventional electrode catalyst accordingto Comparative Example 1 shows an increase in size of the platinumnanoparticles to about 4.8 nm at a temperature of 300° C. This indicatesthat coarsening of platinum nanoparticles occurs due to the increasedtemperature. On the contrary, the electrode catalyst coated with analuminum phosphate-based compound according to Example 1 shows littlecoarsening of platinum particles even under high temperature, and isprevented from coarsening (see Table 1). TABLE 1 Size of Ptnanoparticles in electrode catalyst Temperature (particle diameter; nm)(° C.) Comp. Ex. 1 Ex. 1 Room 2.62 nm 2.62 nm temperature 200° C. 3.50nm 2.70 nm 300° C. 4.80 nm 3.22 nm

Experimental Example 3 Evaluation for Quality of Fuel Cell

The following test was performed to evaluate the quality of a unit cellusing the electrode catalyst according to the present invention.

To both electrodes of the unit cell obtained from Example 1, air andhydrogen were supplied. Then, current density and electric power densitywere measured while varying the voltage applied between both terminalsof the unit cell. The results are shown in FIG. 4.

After the test, the electrode catalyst according to Example 1 shows acurrent density of 0.27 A/cm² and an electric power density of 0.17W/cm² at 0.65V. It can be seen from these results that the electrodecatalyst according to the present invention has quality equivalent tothe quality of a membrane electrode assembly for a general polymerelectrolyte fuel cell, when the amount of platinum supported on theoxygen electrode of 0.25 mgPt/cm² is taken into consideration (see FIG.4).

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, according to the present invention, acompound capable of preventing a catalytic substance for an electrodefrom coarsening is coated onto and/or dispersed in such sites thatcoarsening of the catalytic substance can be inhibited to the highestdegree, thereby improving the structural stability of the catalyticsubstance. Therefore, it is possible to provide a fuel cell havingexcellent lifespan characteristics while not causing any degradation ofelectrochemical quality.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings. On the contrary, it is intendedto cover various modifications and variations within the spirit andscope of the appended claims.

1. An electrode catalyst comprising: (a) a support; (b) metal catalystparticles supported on the support and formed of a catalytically activemetal or metal-containing alloy; and (c) an anti-coarsening compound,which is dispersed in at least one region selected from the groupconsisting of interstitial spaces among the metal catalyst particles andcontact sites between the support and the metal catalyst particles, andhas a coarsening temperature higher than that of the metal catalyst. 2.The electrode catalyst according to claim 1, wherein the coarseningtemperature is a temperature at which crystal particles start to grow.3. The electrode catalyst according to claim 1, wherein metal catalystparticles (b) have a coarsening temperature of 300° C. or lower, andanti-coarsening compound (c) has a coarsening temperature of 300° C. orhigher.
 4. The electrode catalyst according to claim 1, wherein thecatalytically active metal or metal-containing alloy particle is atleast one selected from the group consisting of platinum (Pt), ruthenium(Ru), rhodium (Rh), palladium (Pd), gold (Au), silver (Ag), iridium (Ir)and osmium (Os).
 5. The electrode catalyst according to claim 1, whereinthe catalytically active metal or metal-containing alloy particles havea size (particle diameter) of 1˜10 nm.
 6. The electrode catalystaccording to claim 1, wherein anti-coarsening compound (c) has a higheraffinity to support (a) when compared with its affinity to catalyticallyactive metal or metal-containing alloy (b).
 7. The electrode catalystaccording to claim 1, wherein the anti-coarsening compound is selectedfrom the group consisting of metal phosphates, metal oxides, metalnitrides and metal carbides.
 8. The electrode catalyst according toclaim 7, wherein the anti-coarsening compound is an aluminumphosphate-based compound, zirconium oxide, cerium oxide, silicon oxideor aluminum oxide.
 9. The electrode catalyst according to claim 1,wherein the anti-coarsening compound is coated or dispersed to athickness of 1˜5 nm.
 10. The electrode catalyst according to claim 1,wherein the support is selected from the group consisting of porouscarbon, conductive polymers and metal oxides.
 11. A membrane electrodeassembly comprising: (i) a first electrode having a first catalystlayer; (ii) a second electrode having a second catalyst layer; and (iii)an electrolyte membrane interposed between the first electrode and thesecond electrode, wherein either or both of the first catalyst layer andthe second catalyst layer include the electrode catalyst as defined inclaims 1, the electrode catalyst comprising: (a) a support; (b) metalcatalyst particles supported on the support and formed of acatalytically active metal or metal-containing alloy; and (c) ananti-coarsening compound, which is dispersed in at least one regionselected from the group consisting of interstitial spaces among themetal catalyst particles and contact sites between the support and themetal catalyst particles, and has a coarsening temperature higher thanthat of the metal catalyst.
 12. The membrane electrode assembly (MEA)according to claim 11, wherein metal catalyst particles (b) have acoarsening temperature of 300° C. or lower, and anti-coarsening compound(c) has a coarsening temperature of 300° C. or higher.
 13. The membraneelectrode assembly (MEA) according to claim 11, wherein anti-coarseningcompound (c) has a higher affinity to support (a) when compared with itsaffinity to catalytically active metal or metal-containing alloy (b).14. The membrane electrode assembly (MEA) according to claim 11, whereinthe anti-coarsening compound is selected from the group consisting ofmetal phosphates, metal oxides, metal nitrides and metal carbides.
 15. Afuel cell comprising the membrane electrode assembly (MEA) as defined inclaim
 11. 16. The fuel cell according to claim 15, which is a protonexchange membrane fuel cell (PEMFC).
 17. A method for preparing theelectrode catalyst as defined in claim 1, the method comprising thesteps of: (a) dispersing or dissolving an anti-coarsening compoundhaving a coarsening temperature higher than that of a catalyticallyactive metal or metal-containing alloy into a solvent to provide adispersion or solution; (b) adding a support, on which metal catalystparticles formed of the catalytically active metal or metal-containingalloy are supported, into the dispersion or solution obtained from step(a) so as to be coated with the dispersion or solution, followed bydrying; and (c) heat treating the dried product obtained from step (b).18. The method according to claim 17, wherein the support, on which themetal catalyst particles are supported, is added to the dispersion orsolution of step (b) in a molar ratio of the catalyst particles to theanti-coarsening compound of 1˜5:1.
 19. The method according to claim 17,wherein the heat treating temperature is lower than the coarseningtemperature of the catalyst particles.
 20. A method for preventing metalcatalyst particles supported on a support and formed of a catalyticallyactive metal or metal-containing alloy from coarsening, the methodcomprising: dispersing an anti-coarsening compound having a coarseningtemperature higher than that of the metal catalyst, in at least oneregion selected from the group consisting of interstitial spaces amongthe metal catalyst particles and contact sites between the support andthe metal catalyst particles.